In the context of this specification and its claims, reference will be made to "bevel" gears. This reference is intended to include gears that are generally conical in form and operate on intersecting axes or non-parallel, non-intersecting axes, for example, hypoid gears.
The formation of non-generated bevel gears may be accomplished by a plurality of methods among which are face milling or face hobbing.
Face milling comprises rotating cutting blades arranged in a circle about a cutter and in line with each other. The work piece is held against rotation during cutting. After a tooth is formed the work piece is indexed to the position of the next tooth space and the cutting process is repeated.
Face hobbing also comprises cutting blades arranged in a circle about a cutter, not in line with each other, but in groups, usually pairs. Unlike most face milling processes, in which one cutting blade passes through the tooth slot at a time, face hobbing comprises each group of cutting blades passing through a tooth slot with each blade in the group forming a cut completely along the longitudinal portion of the tooth slot. The face hobbing process is such that more than one blade is present in the tooth slots at any given time. The cutter and the workpiece rotate in a timed relationship to each other thereby allowing continual indexing of the workpiece and continual formation of each tooth slot of the gear. See, for example, U.S. Pat. No. 2,978,964 to Wildhaber.
After the face milling or hobbing has been performed a deburring operation is carried out.
The next operation generally performed after cutting and deburring is heat treating in order to harden the gear. The particular heat treat parameters are selected in order to produce the properties required for the environment in which the gear will be used.
After the face milling or face hobbing operation, deburring and heat treating, the gear is subjected to a hard finishing process. Examples of this process include grinding, skiving or lapping.
Grinding and skiving are correction processes to reshape the gear to a predetermined form. These processes may be utilized to correct dimensional irregularities due to warpage as a result of heat treating.
Lapping is also a semi-corrective process comprising rolling two gear members together, i.e. ring gear and pinion, in the presence of an abrasive. The abrasive is generally composed of an abrasive grit suspended in a fluid material. The object of lapping is to improve tooth contact patterns between the gear members and to improve surface finish.
It is during the operation of the gears that the problem of lapping interference arises. Lapping interference is the result of contact between the lapped portion of a bevel gear and an unlapped portion of a mating pinion. The interference occurs at the sharp corner of the toe end, generally on the convex side (drive side), of the bevel gear due to contact with an unlapped area of the mating pinion. Misalignment at assembly or deflection, for example, could cause the bevel gear to shift slightly relative to the pinion and contact an unlapped area. Lapping interference therefore results from the mismatch of lapped and unlapped surfaces. The results of lapping interference include excessive gear noise, pitting at the interference point or breakage of the gear tooth.
In the production of bevel gears by face milling the problem of lapping interference has been addressed by the inclusion of a secondary finishing operation which removes additional stock, i.e. creates a relief, at the toe end on the convex side (drive side) of the gear. The cutting blade is given a momentary axial thrust and then withdrawn slightly as it enters the toe end of the tooth slot thereby removing additional stock and creating a relief. Since there is no sharp corner remaining at the toe end of the convex side there can be no lapping interference.
However, the axial thrust procedure of face milling does not apply to face hobbing. As previously stated, face hobbing involves more than one cutting blade present in the tooth slots at any given time. It can be seen that providing an axial thrust to a cutting blade entering a tooth slot would mean that the cutting blade or blades already in a tooth slot would also be subjected to the same axial thrust. The result would be gouging of the interior of the tooth slots by those cutting blades.
Therefore, until now, the only way to eliminate lapping interference in face hobbed bevel gears was to include a separate chamfering operation after lapping in which stock material at the toe end of generally the convex side of the gear was removed thereby forming a relief.
It has now been discovered that a relief can be formed during the face hobbing process. This discovery eliminates the additional chamfering process resulting in considerable savings of both time and money.